The main components of a drive train are a drive assembly and transmission. The transmission converts rotational speeds and torques, and provides the tractive power of the drive assembly at an output drive of the drive train. The present invention relates to a method for operating a drive train having a drive assembly designed as a hybrid drive, wherein a clutch is connected between the combustion engine of the hybrid drive and its electric machine, and a further friction element, which is disposed internally or externally to the transmission, and which is designed as a clutch or brake and is serving as a further drive element, is connected between the electric machine and the output drive.
The electric machine of a hybrid drive can be operated as a motor or as a generator. When the electric machine of the hybrid drive operates as a motor, it converts electrical energy into mechanical energy for supplying drive torque. When the electric machine of the hybrid drive operates as a generator, it converts mechanical energy into electrical energy to charge, for example, an electrical energy accumulator of the hybrid drive.
Under certain circumstances it can be necessary for the electric machine of the hybrid drive to exhibit a neutral behavior with respect to the electrical energy accumulator of the drive train, that is, the electric machine neither charges nor discharges the electrical energy accumulator. In this case, the electric machine of the hybrid drive is short circuited.
In particular, if the electric machine is designed as a permanent magnet synchronous machine (PSM), during acceleration of a short-circuited electric machine from a stopped state, short-circuit behavior can be observed in which a relatively high torque must be overcome at the electric machine in order to drive the machine up to a target rotational speed, or to accelerate it, from a stopped state. Then, accordingly, if a drive train with a hybrid drive and a short-circuited electric machine is to be started up, the electric machine strongly brakes at relatively low rotational speeds, which impacts the start-up behavior negatively. To date, there are no known methods for operating a drive train with a hybrid drive that create reliable and comfortable start-up with a short-circuited electric machine.